The invention relates to an apparatus designed to be attached to the bottom portion of a tubing in a well in order to perform tests for determining the characteristics of an earth formation into which the well penetrates and the changes to be expected therein as a function of time.
These tests consist mainly in measuring pressure variations following one or more successive operations of closing and opening the well at the bottom end of the tubing.
To this end, one well test apparatus comprises a test valve mounted at the bottom of the tubing and a measurement assembly including, in particular, a pressure sensor. In addition, the apparatus is designed so as to enable the test valve to be remotely controlled.
In some test apparatuses such as Schlumberger's "PCT full bore" apparatus, a ball test valve is used which is controlled by a pressure pulse sent from the surface, and the results of pressure measurements are stored downhole by recorders until the apparatus is extracted from the well. The information is therefore not immediately available for exploitation.
In order to remedy this drawback, proposals have been made, as described in particular in U.S. Pat. No. 4,678,035 to place a flapper valve above an existing ball valve and to actuate the flapper valve mechanically by means of a wireline assembly suspended on an electrically conductive cable. This wireline assembly then includes measurement sensors that deliver signals which are immediately transmitted to the surface via the cable. Compared with the preceding test apparatus, such test apparatus has the advantage of enabling the results of the pressure, temperature, and/or flow rate measurements performed to be obtained in real time.
Usually, the test apparatus described in patent U.S. Pat. No. 4,678,035 is mounted in a string which already includes a ball valve beneath said apparatus. However, the ball valve is then not used since the well is opened and closed under the control of the flapper valve in the test apparatus described in this U.S. patent.
In addition, the test apparatus described in U.S. Pat. No. 4,678,035 suffers from certain drawbacks.
Thus, if some device such as a perforator gun is conveyed to the bottom of the tubing through the flapper valve, it can happen that while it is being raised, the device causes the valve to close. This can lead to the device being damaged, and also to the flapper valve being damaged, and in the worst cases, it can also lead to the device being jammed inside the test apparatus.
In addition, reopening the flapper valve takes place by releasing the tension exerted on the cable, after equalizing the pressures on opposite sides of the flapper valve via small ducts provided for this purpose. Consequently, a considerable period of time may be necessary after a long period of closure and in the presence of a high pressure difference across the flapper valve.
Finally, when the well is open, measurements are performed while the cable is in a relaxed or slack position, thereby running the risk of damaging the cable.
In order to remedy these drawbacks, proposals have been made to use a ball valve in order to control opening and closing of the well, and to transmit information relating to the fluid situated beneath the valve to measurement means incorporated on a wireline assembly comparable to that which is described in U.S. Pat. No. 4,678,035. The fluid is transmitted via a passage bypassing the ball valve. In order to ensure that well closure is not affected by the presence of this passage, the upstream and downstream portions thereof are normally isolated from each other by a sealing gasket mounted on a sliding sleeve disposed coaxially inside the tubular assembly carrying the ball valve. When the wireline assembly is in place, the upstream and downstream portions of the passage are put into communication by exerting traction on the cable, and this has the effect of displacing the sleeve upwards and of placing the upstream and downstream portions of the passage on the same side of the sealing gasket carried by the sleeve.
Although this solution has the advantage of being simpler than the preceding solution and of avoiding the drawbacks associated therewith, it nevertheless suffers from a major difficulty. Given that the sleeve makes contact with the wall of the tubular assembly via large diameter sealing gaskets (e.g. about 65 mm in diameter), displacement of the sleeve requires a traction force to be applied which can be very high when the pressure difference across the gaskets reaches large values. Given that the pressure difference may reach or even exceed 500 bars, the traction force that needs to be exerted on the cable in order to maneuver the sleeve may exceed 400 kg. Given the relative weakness of the cable, there is a high risk of it breaking.
The object of the present invention is to provide a well test apparatus operating on a principle analogous to that of the last-described solution above, but having a special structure for opening the passage via which the space situated beneath the ball valve communicates with the measurement means in the wireline assembly, enabling the passage to be opened by a smaller traction force that does not endanger the mechanical strength of the cable.